Blow-molded plastic containers for containing liquids at elevated pressures are known and have found increasing acceptance in the beverage industry. Such containers have particular advantages in that they have considerably less weight than glass containers, are generally less subject to breaking during handling and transportation and may be relatively easily manufactured. Moreover, the materials used in their manufacture may also be recycled after use. In general, these types of containers are most convenient for use as one way disposable containers.
Although such containers are particularly well suited for use in the beverage industry, plastic bottles of this type are subject to a number of structural and functional criteria which have presented many problems which have not been adequately solved. Solutions to the problems offered by the prior art have yielded bottles which are still not entirely satisfactory.
Several types of containers exist in the known art which include integral bases with molded bottom configurations. Examples of these types of containers are found in U.S. Pat. No. 3,403,804 to Columbo entitled "Blow Molded Bottle of Flexible Plastic"; U.S. Pat. No. 4,249,667 to Pocock, et. al. entitled "Plastic Container with a Generally Hemispherical Bottom Wall having Hollow Legs Projecting Therefrom"; U.S. Pat. No. 3,935,955 to Das entitled "Container Bottom Structure"; U.S. Pat. No. 4,108,324 to Krishnakumar, et. at. entitled "Ribbed Bottom Structure for Plastic Container"; U.S. Pat. No. 3,871,541 to Adomaitis entitled "Bottom Structure for Plastic Containers"; U.S. Pat. No. 3,598,270 to Adomaitis, et. at. entitled "Bottom End Structure for Plastic Containers"; U.S. Pat. No. 5,024,340 to Alberghini, et. at., entitled "Wide Stance Footed Bottle"; U.S. Pat. No. 4,867,323 to Powers, entitled "Blow-molded Bottle with Improved Self-Supporting Base" and U.S. Pat. No. 4,978,015 to Walker, entitled "Plastic Container for Pressurized Fluids". While there are structural differences in the containers disclosed by these patents, they all share a common feature in that these containers have feet with contact surface edges which are essentially of a uniform radius with respect to the bottle circumference. These containers are generally acceptable; however, they are still susceptible to stress cracking and there still exists a need for a container of this type which may be manufactured with a minimal amount of material in the base; is capable of withstanding internal pressures; is resistant to stress cracking; will stand upright without rocking and which can be manufactured in a high-speed bottle manufacturing environment.
In existing one piece bottle bottom construction, three general problems have been identified in the art. Initially, such plastic bottles have not had enough bottom strength to withstand the impact of falling from moderate height on to a hard surface when filled with a carbonated beverage. Further, because the bottles are often subjected to extreme temperatures, it has been found in some designs that the bottom of the bottle inverts or otherwise distorts producing a bottle known in the industry as "rocker" where the bottle wobbles in transportation or display and is otherwise unstable. Finally, another problem is the stress cracking of such bottles, especially under extremes of temperature or pressure or when exposed to any stress cracking agent during filling, handling or subsequent transportation. The problems with these types of bottles are due to design limitations and to material characteristics and flaws which are often exaggerated in a high speed bottle manufacturing environment, particularly where the plastic preform may be improperly heated, insufficiently stretched, inadequately oriented and/or a combination of these defects. Simply stated, these bottles are often incorrectly blown.
PET is a plastic polymer material with a combination of properties which are particularly desirable for the packaging of carbonated beverages. These properties include flexibility, toughness, clarity, creep resistance, strength, and high gas barrier. Furthermore, because PET is thermoplastic, it can be recycled by the application of heat and is therefore environmentally attractive.
The processing of the container of the present invention involves the injection molding of PET into what is commonly referred to as a "preform" and blow-molding the preform into the container. In such a process, biaxial orientation is introduced into the PET by producing stretch along both the length of the bottle and the circumference of the bottle. In stretch blow molding, a stretch rod is utilized to elongate the preform and air or other gas is blown into the preform and radially stretches the preform, both of which happen essentially simultaneously. Prior to blow-molding, the preforms are preheated to the correct temperature, generally about 100.degree. C., but this varies depending on the particular PET or other plastic material being used.
In the various known processes for manufacturing plastic blow-molded PET bottles, there are certain parameters which must be carefully controlled in order to produce commercially acceptable containers on a reliable basis. These process parameters are generally referred to as the "process window" and include, in addition to the temperature (i.e. heating and cooling), dwell time in the mold, stretch force of the rod and the pressure of the air or other gas blown into the container. Of those parameters, the temperature and dwell time in the mold, generally referred to as the temperature profile, are often thought to be most critical, particularly, in containers with integral self-supporting bases. In manufacturing these bottles in a high speed manufacturing environment, slight variations, minor modifications or aberrant fluctuations in any one of these parameters often leads to unacceptable results. In these situations the process window is said to be narrow in that there is little, if any, tolerance for even the slightest change in these parameters.
For example, it is known in the art that temperature and temperature profile of heating the preform is important to achieve the intended distribution of material over the bottom wall during forming. It is also well known in the art how to alter such a temperature profile to produce an acceptable bottle once the design of the mold is known. Once the PET preform is in the desired temperature it is secured by its neck in a mold which has a cavity of the desired bottle shape. A stretch rod is introduced into the mouth of the bottle to distribute the material the length of the bottle and to orient the molecules of PET longitudinally. Simultaneously, air is blown into the bottle from around the stretch rod to distribute the material radially to give the radial or hoop orientation of the PET.
As the newly formed bottle expands, the exterior surface of the bottle comes into contact the mold interior surfaces which are cooled to a temperature which may be substantially less than the preheat temperature of the mold, via water-cooling or other similar means. Contact of the heated and stretched plastic with the cooler mold surfaces causes the biaxially oriented PET to rapidly cool. Preferably, it is desirable to have the bottle walls contact all the mold surfaces nearly simultaneously, in order that the cooling is uniform. After sufficient cooling has taken place, the mold is opened and the finished bottle is removed.
During blow-molding, the preform plastic first contacts the apex and rib portions of the mold and then stretches into the feet and to the bearing surfaces. As a result, the plastic cools in the apex and rib area and reduces the stretchability of the plastic. The effect of this non-uniform cooling is a greater wall thickness in the apex and ribs which, in turn, requires an increase in dwell time in the mold in order to stretch the plastic sufficiently to reach the outermost edge portions of the bearing surfaces.
It is well recognized that the utility of plastics, in general, and specifically of PET as a material for blow-molded containers is dependent upon the form in which it exists in a solid state. For example, solid PET exists in three basic forms: amorphous, crystalline, and biaxially oriented. Each form has characteristics which make it suitable for use either in the preform or in the blown bottle, but rarely in both.
PET in the amorphous state is formed when molten PET is rapidly cooled to below approximately 80.degree. C. It appears clear and colorless and is only moderately strong and tough. This is the state that preforms are in prior to being injection molded. Crystalline PET is formed when the molten PET is cooled slowly to below 80.degree. C. In the crystalline state, PET appears opaque, milky-white and is brittle. Crystalline PET is stronger than amorphous PET and because it is strong, badly formed bottles will result from the blow molding process if significant amounts of crystalline PET are present in the preform.
Oriented PET is formed by mechanically stretching amorphous PET at above 80.degree. C. and then cooling the material. Biaxially oriented PET is usually very strong, clear, tough and has good gas barrier properties. It is generally desirable in order to obtain sufficient biaxial orientation that the amount of stretch being applied to the amorphous PET be on the order of at least 3 to 1.
Finally, while biaxially oriented PET is exceptionally clear and resistant to stress cracking, non-biaxially oriented, crystalline PET is neither clear nor resistant to stress cracking. Further, amorphous PET, although clear is not resistant to stress cracking. Thus, it will be appreciated that in the design and processing of blow-molded plastic containers made of PET, it is desirable to minimize or eliminate the presence of any crystalline PET material in a preform as well as to obtain the maximum biaxial orientation possible in the blown bottle.
Blow-molded bottles formed from injection molded preforms tend to have a particularly acute stress cracking problem in two areas. The first problem area is the bottom portion of the bottle which includes and lies adjacent to the nib remaining on the preform from the sprue or "gate" through which the molten polymer is injected into the preform mold. This gate area is manifested in the blow-molded bottle by a clouded circlet at or very near the center of the bottle bottom. In prior art bottles, this gate area contains far less biaxial orientation than is present in the bottle sidewall or in the remainder of the bottom. As a result of this deficiency, the gate area of a bottle blow-molded from an injection molded preform is more apt to fail under stress than other areas of the bottle sidewall or bottom. The second problem areas which are susceptible to stress cracking are found at or near the transition surfaces between the bottle ribs and where the contact surfaces intersect with the container sidewalls. Stress cracking typically occurs in these areas because of improper distribution of plastic materials, or from insufficient stretch and orientation, or both. Often these problems are due to errors which occur during the processing of such containers, particularly in a high speed bottle manufacturing environment where the process window may be narrow because of the critical relationship between the manufacturing parameters. These errors cause the plastic molded materials to be structurally weak in specific areas which when coupled with the high internal pressures of a filled container and bending moment of the plastic, frequently lead to bottle failure. Stress cracking can occur due to a combination of these problems and is exacerbated particularly under the extreme conditions experienced in the transportation and storage of pressurized containers and especially in geographical areas where ambient temperatures can exceed 100.degree. F.
From the foregoing problems inherent in known prior art bottle designs and manufacturing, it can be seen that it would be desirable to provide a bottle design which may be made with maximum stretch and orientation and minimum thickness in the bottom portion. It will also be appreciated that it would be desirable to provide a bottle which has a shorter process time and simultaneously, a larger process window particularly suited to a high speed bottle manufacturing environment. Finally, it also be appreciated that it would be desirable to provide a bottle which uses less plastic materials but is more resistant to stress cracking than known prior art bottles.
Accordingly, it is an object of this invention to provide a plastic bottle in which the manufacturing process window is enlarged.
It is another object of this invention to provide a plastic bottle in which the plastic material is distributed in a more uniform manner throughout the bottle and particularly in the bottom portion.
Still another object of this invention is to provide a bottle with better standing capability.
Yet another object of this invention is to provide a bottle having improved stability, improved resistance to stress cracking as well as providing a bottle with a reduced weight resulting in a cost saving of material used.
From the subsequent description and claims taken in conjunction with the accompanying drawings, other objects and the advantages of the present invention will become apparent to those skilled in the art.